Electroplating and mechanical plating are conventional methods used in large-scale economical coating of materials, such as heavy metals, alloys, metal compounds and composites on industrial components. Some of the materials which are plated in large quantities include metals such as zinc, cadmium and chromium; alloys such as zinc-cadmium, zinc-chromium of various compositions; compounds such as cadmium oxide, zinc sulphide, zinc-telluride and cadmium telluride; and composites such as zinc-carbon. Some of these materials are used as protective coatings to prevent early corrosion in various ambients. Others are used as lubricous coatings on fasteners. Cadmium-telluride, zinc-sulphide and zinc-telluride are semiconductor materials which are used in optoelectronic device applications. Cadmium oxide is used in Nickel-Cadmium batteries. Cadmium coating is preferred when protection in marine environments, as well as high lubricity, is of prime importance. Hard chromium is preferred when appearance, corrosion resistance and scratch protection is desired simultaneously. Zinc is used for plating fasteners and galvanizing steel rolls. Recently, zinc-cadmium alloys were found to be superior to cadmium in corrosion properties in all environments and are being substituted for cadmium to reduce the use of toxic cadmium. Zinc, cadmium and chromium are electroplated in the defense, aerospace and automobile industries on a large scale.
The conventional electroplating and mechanical plating methods can use large quantities of hazardous toxic chemicals such as cadmium, hexavalent chromium, sodium cyanide and chromic acid, and may discharge environmentally unacceptable effluent into the waste water streams. Large quantities of precipitated heavy metal sludge produced in these plating operations are usually disposed of at a selected number of government regulated land sites. Industrial electroplating waste has been reported to be the largest single source of contamination of the natural water streams in the United States. The U.S. Environmental Protection Agency (USEPA) and the U.S. Department of Defense (USDoD) are continuously seeking environmentally acceptable technologies which would minimize toxic waste generation at its origin. A key aspect of the present invention is its ability to provide an environmentally and occupationally safe plating method which eliminates liquid chemicals and minimizes waste.
A second problem encountered with prior art devices relates to the configuration of the source material. In this regard the present invention is distinctly different from conventional glow discharge plating or ion plating. In prior ion plating methods a metal is melted in a hot crucible or on a hot filament wire to generate coating vapors. This method is demonstrated in U.S. Pat. Nos. 4,116,161 to Steube and 3,329,601 to Mattox. Using a molten metal source places restrictions on the location, size, shape and the amount of the source available in the apparatus, and adds to the complexity of fixturing as well as cost of the apparatus. The complexity in molten source fixturing is increased as the area and/or the number of parts to be coated in a batch is increased and as the plating rate is increased. Through the present invention, these restrictions are either eliminated or substantially reduced. The added flexibility in source configuration allows for simplified, lower-cost technology for large scale manufacturing.
In addition, after a session of deposition the chamber is often left with a great deal of extraneous material deposited upon its surfaces. Some devices have sought to minimize this condition by directing the vaporized material toward the targeted surface. This technique is effective in deferring, not deterring extraneous deposition. Eventually, even deposition chambers using this directional process must be cleaned. In addition, regardless of the cleaning process, the recovered material is usually of a lesser quality due to impurities and such. The present invention offers two distinct approaches to address these chamber and recovery problems. One serves to prevent extraneous deposition, while the other is an almost effortless method for recovering any extraneously deposited material on chamber surfaces.
In the field of sublimation processes other problems have been known to exist. Indeed, sublimation of semiconductor materials has been used in the past, as evidenced by U.S. Pat. No. 4,207,119 to Tyan, for growing films on flat surfaces. However, this has been limited to close-space sublimation, or CSS. As the name implies, this process is useful only for flat substrates in close proximity to a flat source. This arrangement was found necessary to grow layers with suitable photovoltaic properties on the substrate. The high pressures at which CSS operates--500 milliTorr to 1 Torr--causes the deposited material to form clumpy, powder-like layers as the distance between the substrate and the source material increases. These type of layers possess poor photovoltaic properties, and poor adhesion qualities. Tyan teaches placing a limitation on the source to substrate distance as the square root of the smaller of the surface area, or preferably between 5 to 5 mm. This limited space does not of course allow for operation in a glow discharge mode, as taught by the present invention. Two articles, "The Use Of Close Spacing In Chemical Transport Systems For Growing Epitaxial Layers of Semiconductors", by F. H. Nicoll and "CulnSe.sub.2 and CdTe: Scale-up For Manufacturing" by K. Zwiebel and R. Mitchell, discuss close-space sublimation in much greater detail, including many of the problems of CSS. The present invention eliminates the limitation of close spacing between the source and substrate. In addition, the present invention does not limit the size and shape of either source or substrate, as discussed earlier. Uniform coatings are possible with the present invention on in regular substrates, as well as flat substrates.
The present invention, in its various methods and apparatus, recognizes and addresses these and other problems and overcomes many limitations encountered by those skilled in the art. Many devices and procedures have taught the use of a molten source, but none have realized the advantages of a solid source material as in the present invention. Until the present invention, it has not been known to take the present approach of utilizing sublimating metal and semiconductor materials for plating processes in a glow discharge, despite the long felt need for environmentally and occupationally safe methods and apparatus, and the existence of the necessary implementing arts. Problems such as waste of source material exist in the field of vapor deposition, but such problems have not been adequately addressed by those skilled in the art. While material sublimation is a well known physical property of some elements and compositions, its value in the field of deposition has gone virtually unnoticed because those skilled in the art failed to address the aspects of adequate adherence, variable source configuration--size, placement, shape, etc.--and especially material waste control. The recognition by the present inventor that the problems encountered in conventional deposition could be solved by using sublimating materials lead to the present invention. The prior art has taught away from the present invention by stressing molten source reservoirs rather than solid source materials. Other areas of teaching away by the prior art relate to the methods of recovery. While prior art systems employ tedious scraping and cleaning techniques to rid the chamber of extraneous source material, the present invention again takes advantage of the sublimating property of certain materials to recover/reclaim a pure, reusable source. Rather than supplying a system which affords only an incremental increase in performance and design over the prior art, the present invention utilizes techniques which were not previously considered to achieve leaps in performance compared to the prior art.